This invention relates to gearbox assemblies for electric power steering assemblies of the worm and wheel type.
Electric power steering systems use an electric motor to produce an assistance torque that is applied to a rotating part of the steering system. In a conventional arrangement this torque assists the driver in turning the wheel. Because motors work best at relatively high speeds and because compact motors produce relatively low torques, the connection between the output of the motor and the steering column is usually through a reduction gearbox.
The most widely used type of electric power assisted steering reduction gearboxes are of a relatively simple worm and gear configuration. The gearbox assembly typically comprises a gearbox housing which houses a worm shaft and a gear wheel. The worm shaft is connected to the output of an electric motor (shown in part at the far right). The motor may be secured to an end face of the housing or even located within the housing. The worm shaft is supported by a main bearing assembly at an end closest to the motor and a tail bearing assembly at an end furthest from the motor, both bearing assemblies typically comprising ball bearing elements supported within an inner bearing race that is threaded onto the worm shaft and an outer bearing race that is secured to the housing. The function of the bearings assemblies is to allow the worm shaft to rotate whilst to a certain degree limiting axial and radial movement as will be explained. The gear wheel is connected to an output shaft of the gearbox and located so that teeth of the gear wheel engage teeth of the worm shaft.
It is known that the speed reduction gearboxes used in Electrical Power-assisted Steering (EPS) apparatus are prone to rattle due to external torsional vibrations acting at their output shafts. These vibrations originate at the road wheels due to surface roughness or unbalanced wheels. Alternatively, mechanical noise can arise from sudden torque reversals applied at the steering wheel by the driver. The main rattle sites in a worm and wheel gearbox are at the engagement of the worm and gear teeth and at the “main” ball bearing, closest to the motor, which axially locates the worm shaft.
A well-known solution to the rattle is the so-called “Sprung Worm” mechanism. In the “Sprung Worm” mechanism, a biasing means such as a leaf spring applies a biasing force that urges the worm shaft into engagement with the wheel gear. The biasing means requires a small amount of radial movement of the worm shaft and this is achieved by allowing it to pivot around its axis in the plane of the gearwheel by a small angle (typically less than +/−0.5 degrees) either side of its nominal position. The pivoting is normally around an axis which is nominally at the centre of the main bearing. This movement is typically controlled by a specially configured tail bearing that is allowed to move by small amounts (typically less than +/−0.5 mm) and typically by the main bearing having sufficient internal axial clearance between its balls and the sides of its race grooves to permit a small articulation (i.e. tilting) angle which is typically less than +/−0.5 degrees.